A rod of length 39.0 cm has linear density (mass-per-length) given by the following equation, where x is the distance from one end.
ë = 50.0 g/m + 21.0x g/m2

(a) What is its mass?
(b) How far from the x = 0 end is its center of mass?

I think you would plug in the 39cm for the x in the equation, but change it to meters first of course. The problem is how do you just get it in grams then because the units you get are g/m. I'm not sure how to even start with the center of mass.